In the color television system used in Europe referred to as PAL, standing for phase alternate line, the frequency of the color subcarrier is such that there are an integral number of quarter cycles plus a small fraction of a cycle occurring during each line scanning interval. During a field, the fractions accumulate to a value of two quarter cycles so that the subcarrier has a phase at the start of the next field that is 180.degree. different from the phase it had when it started scanning the field just completed.
This reduces the noticeability of dots formed by the presence of the color subcarrier in an image formed on the screen of a television receiver for the following reason. If there were an integral number of quarter cycles occurring during a line scanning interval, the dots produced at sharp vertical edges in a single field would be visible but not particularly noticeable, but when dots are formed in a succession of fields, they would appear to crawl so slowly in a diagonal direction that the eye would have a strong tendency to follow them. Therfore, instead of using a subcarrier frequency of 1135/4 times the line scanning frequency Fh that would provide an integral number of quarter cycles during a line scanning interval, the frequency of the subcarrier in the PAL system was increased by a 25 cycle per second offset so as to attain a non integral number of quarter cycles along a line. Although the dots are still present, they move more rapidly in a diagonal direction so that the tendency of the eye to follow them is significantly reduced, thereby making them less noticeable.
Whereas this improves the images, it creates significant problems in digital signal generators, processors, time base correctors and other digital equipment and requires single sideband heterodyne circuits or phase locked loops because a lock to the color subcarrier can be made at field intervals only when the accumulated fractions equal an integral number of quarter cycles.